Not applicable.
1. Field of the Invention
The present invention relates to a method for the production of a multi-layer identity card of plastic.
2. Description of the Related Art
The invention relates to a method for the production of a Multi-layer identity card of plastic. Such identity cards are widely employed as check or bank cards or as an authorization means for access to mobile radio communication systems. They comprise at least one card core layer, which is printed on at least one side and a covering layer applied to the printed the side of the card core layer. These card layers are joined together in a laminating process.
The covering layer, which is usually made transparent, has an adhesive coating thereon adhering to the printed card core layer, and serves inter alia for protecting the print and also as a means bearing additional information, which for instance is applied in the thermotransfer printing method.
For the card layersxe2x80x94both the card core layers and also the covering layersxe2x80x94various different materials come into question: for instance polyvinyl chloride (PVC), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET, PETG and PETF). The selection of the materials and accordingly of the card structure is dependent on different factors. These factors are for instance: the desired mechanical properties of the identity card to be manufactured, its behavior at high and low temperatures, the question of printability, the question of laser inscription and furthermore economic and ecological determinants.
Dependent on which card structure is selected, different laminating parameters (temperature, pressure and time) are predicative. Same are dependent more especially on which material is utilized for the card core layers.
It is known to apply an adhesive coating (adhesive promoter, adhesive primer) on the card core layer prior to printing, which is to ensure improved adhesion of the printing ink, i.e. printing takes place on an adhesive coated card core layer and not directly on the card core layer. This procedurexe2x80x94printing on an adhesive coated card core layerxe2x80x94is more particularly necessary in the case of so-called digital printing on plastic foils. In the case of digital printing the printing image to be applied to the plastic foil to be printed is temporarily held, that is to say with the aid of selective electrostatic charges, on the printing roller. This digital printing method, which is for example employed in lasers printers, has now only been employed for a short time so far for printing plastic foils for the manufacture of identity cards. The printing inks and printing parameters applicable here are different to those in printing methods (offset litho and screen printing) employed so far in a classical manner, in the range in question, the necessity of printing on an adhesive coated card core layer being the reason.
In the case of lamination of a multi-layer identity card formed in this fashion there is the following problem in the case of relatively high lamination temperatures (over 124xc2x0 C.): above a certain lamination temperature there is an undesired displacement (shift or drift) of the adhesive coatings including the printed layer sandwiched between them in relation to the covering layer and the: card core layer. Adhesive coatings customarily employed form a sort of lubricating film between the card core layer and the covering layer. This phenomenon is to be attributed to the rheological properties of the materials utilized.
One object of the invention is to manufacture a multi-layer identity card of the type initially mentioned in such a manner that the above mentioned displacement does not take place.
In accordance with the invention adhesive coatings are employed, which contain at least one additive, which increases friction between the covering layer and the card core layer so that the formation of a lubricant film is prevented. As additives silica (SiO2), silicates and calcium carbonate (CaCO3) have proved to be particularly effective. The flow behavior of the adhesive coating may in this manner be favorable influenced during further use under pressure and as a raised temperature in lamination.
The adhesive coatings themselves are preferably in the form of a thermoplastic adhesive as for example on the basis of polyamides, polyesters, or polyurethanes or the copolymers thereof In this respect in one embodiment only silica or only silicate or only calcium carbonate as an additive is mixed with the adhesive formulations. In an alternative embodiment there is a provision such that a mixture of two or more additives is incorporated in the adhesive formulations. The surfaces of the additives employed may furthermore be chemically modified so that for instance embedding of the additives in the material of the adhesive coating is improved. Thus for instance in the case of the use of silica (SiO2) as an additive silica with different silane coatings is employed. The silica may be in the form of naturally occurring silica such as sand, quartz or quartzite or synthetic silica.
As a silicate talcum is preferably employed. As calcium carbonate naturally occurring calcium carbonates, such as chalk or limestone or synthetically precipitated calcium carbonates come into question.
The percentage by weight of the additives in the adhesive coating amounts to at least 0.1% and at the most 60%.
The additives employed possess a comparatively large surface area related to their weight. The mobility of the chain molecules of the adhesive coating is reduced by the additives. Furthermore, there is an addition of polymer segments of the adhesive coating on the surface of the additives so that furthermore adjacent polymer segments of the adhesive coating are limited in their mobility. All in all it is possible to say that owing to the presence of the additives the theology of the adhesive coating is so altered as regards the lamination parameters that the viscosity of the adhesive coating is increased so that it is favorable as regards the present application.